1. Field of the Invention
The present invention relates generally to olefin-based crosslinked thermoplastic elastomers and a process of preparation thereof. More particularly, the present invention relates to olefin-based crosslinked thermoplastic elastomers made from an olefin-based resin, an ethylene-propylene-diene rubber and an ethylene-octene copolymer and having improved properties, and to a process of preparation thereof.
2. Description of the Related Arts
Olefin-based thermoplastic resins modified with elastomers such as rubbers to impart elasticity, or elastomers modified with thermoplastic resins to render having processability are extensively developed due to their good physical properties and good processability. These types of materials are designated as thermoplastic elastomers because they exhibit thermoplasticity as well as elasticity. They are advantageous in that they can be directly molded by using extrusion, injection and press molding process without crosslinking-aging step, which is required for common rubber to be molded. For this reason, they replace a wide range of rubbers needing processability and thermoplastic resins needing elasticity. In particular, olefin-based crosslinked thermoplastic elastomers, unlike styrene-based copolymers or urethane-based thermoplastic elastomers, show a wide range of physical and processing characteristics depending on the degree of crosslinking of elastomers incorporated therein. This renders them applicable to a wide range of uses and have highest market share. Various researches have been made to develop new crosslinking systems which can be advantageously used for olefin-based crosslinked elastomers having a same composition. in general, crosslinked thermoplastic elastomers are classified into three groups: no cured thermoplastic elastomer (hereinafter referred to as TPO) wherein rubbers are incorporated without a crosslinking agent; partially crosslinked thermoplastic elastomers (hereinafter referred to as TPR) wherein rubbers are partially crosslinked into the thermoplastic resin; and completely crosslinked thermoplastic elastomers (hereinafter referred to as TPV) wherein rubbers are completely crosslinked into the thermoplastic resin.
The degree of crosslinking affects the size and fine dispersion of rubber particles in crosslinked thermoplastic elastomers, consequently affects elastic characteristics such as tensile strength, permanent tension set, permanent compression set and et al. In more detail, in cases of TPO having no crosslinking of rubber and TPR, there are limitations in obtaining the finely dispersed rubber particles by shear stress. Especially, this becomes more severe as the amount of rubber increases. For this reason, materials requiring elasticity and thermoplasticity should have been made by the dynamic vulcanization in which cured rubbers are pulverized by shear stress during the mixing.
Many developments have been made for crosslinked thermoplastic elastomers. For example, commercial olefin-based crosslinked TPE may include TPR.TM. by UniRoyal, USA (U.S. Pat. No. 3,758,643), which is the first commercial product, and Santoprene.TM. by Monsanto, USA (U.S. Pat. No. 4,311,628). As crosslinked thermoplastic elastomers using peroxides as a crosslinking agent, Milastomer.TM. by Mitsui Petrochemical, Japan (U.S. Pat. No. 4,785,045).
TPR.TM. and Milastomer.TM. are crosslinked hermoplastic elastomers in which a combination of polyolefin resin and ethylene-propylene rubber (hereinafter referred to as EPR), or of ethylene-propylene-diene rubber (hereinafter referred to as EPDM) is partially crosslinked by using peroxide crosslinking agents. When peroxide crosslinking agents are used for the crosslinking of rubbers, crosslinking rate and reactivity are good. On the other hand, peroxide crosslinking agents can not be used in a sufficient amount, since polyolefin is decomposed by the crosslinking agents. Therefore, there is a limitation in improving the elastic characteristics such as permanent compression set or permanent tension set. Moreover, because the decomposition of polyolefin resin is induced during the crosslinking of rubbers goes on, mechanical properties and stability of system are deteriorated.
Santoprene.TM. is a crosslinked thermoplastic elastomer by completely crosslinking a combination of polyolefin and EPDM using a phenolic crosslinking agent. Since a tin-based crosslinking accelerator is used to increase the rate of crosslinking, the final product has odor and brownish color, and is apt to absorb moisture. Although the degree of crosslinking can be increased since only EPDM rubber is used, the remaining crosslinking agent may cause a excessive crosslinking during molding process, consequently the mechanical properties such as tensile strength and elongation, and permanent compression set are decreased.
Accordingly, olefin-based crosslinked thermoplastic elastomers free from the problems of the conventional TPE has been needed.